This invention relates to a control system for vehicle safety device such as an air bag.
As disclosed in U.S. Pat. No. 4,950,914, a control system for an air bag comprises a piezoelectric device (acceleration sensor), and acceleration evaluation means. signal (sensor signal) from the piezoelectric device represents the acceleration and deceleration of a vehicle. The acceleration evaluation means inputs the sensor signal at a predetermined cycle (i.e., at predetermined time intervals), and integrates it, and compares this integral value with a threshold level. This integral value represents a change in the vehicle speed, and increases in the deceleration direction during vehicle collision. When the integral value increases in the deceleration direction and exceeds the threshold level, the acceleration evaluation means judges that a collision has occurred, and outputs a trigger signal, to expand the air bag.
In the control system of the above U. S. Patent, failure or fault of the piezoelectric device can be judge. More specifically, the control system further comprises a test pulse output means for outputting test pulses to the piezoelectric device, and failure judgment means for checking an output level of the piezoelectric device responsive to the test pulse to judge whether or not the piezoelectric device has failed.
In the control system of the above U. S. Patent however, the failure judgment can be effected only when the vehicle is stopped because when a test pulse is supplied to the piezoelectric device, the piezoelectric device outputs a signal unrelated to the acceleration and deceleration of the vehicle. Therefore, the acceleration evaluation means cannot make a proper collision judgment.
Referring to other related art, U.S. Pat. No. 4,873,452 discloses a control system for a vehicle safety device having a failure judgment function for an acceleration sensor, which is similar to that disclosed in the first-mentioned U.S. Pat. U.S. Pat. No. 4,836,024 discloses a control system for a vehicle safety device which has two acceleration sensors, but does not have a failure Judgment function.
One of the inventors of the present invention is a coinventor of a U.S. patent application (Ser. No. 642,951) filed Jan. 18, 1991. This application discloses a technique in which test pulses are simultaneously supplied to two acceleration sensors, respectively, to effect the failure ,judgment of each of the two acceleration sensors.